1. Technical Field of the Invention
The present invention relates to solid-state image pickup elements such as color image sensors that have improved resolution and S/N ratio, and solid-state image pickup apparatuses using the same.
2. Conventional Technology
In recent years, a variety of solid-state image pickup apparatuses that use solid-state image pickup elements such as image sensors to obtain moving pictures or still-frame pictures have been developed. A color image sensor includes pixels that are composed of photodiodes and transistors, and the pixels are two-dimensionally arranged in an array to obtain two-dimensional images.
Pixel signals obtained by the photodiodes of the respective pixels are retrieved through the transistors. In this case, for example, all the pixels in one line are simultaneously selected, and supplied to a column reading circuit in the form of voltages or currents. Further, the pixel signals are sent from the column reading circuit to a column selection circuit, where the pixels are selected one by one and output.
In the color image sensor, pixels that respond to different wavelengths (colors) are alternately disposed, and color components are detected with a plurality of adjacent pixels of different colors. For example, filters of R (red), G (green) and B (blue) colors are appropriately disposed for each of the pixels to realize color imaging.
A so-called “Bayer” arrangement is often used as a pixel arrangement when, for example, a single plate type image pickup system is used. In a Bayer arrangement, green segments are arranged in a checkered pattern, and red and blue segments are alternately disposed in lines sequentially. In other words, although green segments exist at every other pixel in each line, green segments exist at every other pixel only in each odd numbered line, and blue segments exist at every other pixel only in each even numbered line.
Therefore, in this case, in one line, pixels are arranged in green, red, green, red, . . . , and in the next line, pixels are arranged in blue, green, blue, green, . . . There are a variety of other methods as to in what order color pixels are disposed and how colors are selected, but they are similar to one another in that color pixels are alternately disposed.
FIG. 5 is a block diagram of a conventional solid-state image pickup apparatus that uses the image sensor described above.
An image sensor 1 sequentially outputs analog pixel signals. An amplifier circuit 2 adjusts clamp levels according to optical black, and amplifies input pixel signals to output levels that are within a specified range. Outputs of the amplifier circuit 2 are converted into digital signals by an ADC (analog-to-digital converter) 3, and then supplied to a signal processing circuit not shown.
In this manner, pixels in each line are output one by one from the image sensor. Therefore, for example, when a Bayer arrangement is adopted, green pixels and red pixels are alternately output in one line, and green pixels and blue pixels are alternately output in the next line.
However, the transmittance of green filters is different from the transmittance of red filters or blue filters, and output levels of the solid-state image pickup element are higher in green pixels than in red or blue pixels. If the gain of the amplifier circuit is set according to green pixels, the output levels of the amplifier circuit for red pixels and blue pixels become relatively small, and thus the dynamic range of the ADC is not fully utilized, and their resolutions lower.
It is possible that the amplification of the amplifier circuit is changed for each of the colors; but in this case, the amplification needs to be changed for each of the pixels, which requires an extremely fast control and therefore is not practical.
The present invention is made in view of the problems described above, and one object is to provide a solid-state image pickup element that can set optimum amplifications for outputs from a sensor according to colors to thereby improve its resolution and S/N ratio, and a solid-state image pickup apparatus using the same.